I don't know if this will help or not, but we used to have shooting mitts in the military, and I imagine most other militaries did as well. It doesn't look like anything that I have ever seen. The mitts featured a single trigger finger. I found a picture of a more modern mitten as well as an older pair. These are military, but years ago, a lot of equipment was adapted from or patterned after what the military was using.
I cannot say with certainty, but I think he might have got a pair of leather welding mittens and used them for shooting.